(i) Field of the Invention
present invention relates to an image input device and method for converting characters and images to reading digital data, particularly to an image input device and method provided with a linear image sensor and a sub-scanning mechanism for scanning images of line units inputted to the linear image sensor in a perpendicular direction to the lines to read characters and images.
(ii) Description of the Related Art
In a conventional image input device generally called an image scanner device, a linear image sensor is used as an image pickup element of an image input mechanism section, and partial images of line units obtained by the linear image sensor are successively combined in a perpendicular direction to lines to input a two-dimensional original image by a sub-scanning mechanism. For example, as disclosed in Japanese Patent Application Laid-Open No. 291259/1987, this type of image input device is constituted such that the image input mechanism section provided with the image pickup element is supported by a stand, and the like. In the image input device, an original image surface is distant from the image input mechanism section, no special lighting is performed for the image scanner device, and the lighting of the original surface is secured by natural light or the lighting by a fluorescent lamp for use as a usual ceiling lamp. However, in the lighting system, when a speed for incorporating the image is increased, influences such as an illuminance change (flicker) with an elapse of time generated in the fluorescent lamp are exerted, and a phenomenon of transverse streak generated in the image at a flicker period occurs. A method of reducing adverse influences such as the flicker by the change of lighting light quantity is disclosed in Japanese Patent Publication No. 19311/1989.
The method described in the Japanese Patent Publication No. 19311/1989 comprises disposing a plate member provided with a reference reflection section with an optical density as a reference in the vicinity of an original, and disposing a light receiving element in which a plurality of light receiving portions are arranged so that some portions detect image information from the original and other portions detect lighting information from the reference reflection section. The light receiving element is scanned in a perpendicular direction to the arrangement direction of the light receiving portions, and an image signal and a lighting signal are switched during the scanning of one line so that these signals are alternately outputted. Thus, lighting signals exist in the beginnings of one-line scannings. Then, a peak hold circuit controls AGC circuit to adjust levels of lighting signals of subsequent lines such that they agree with the level of a first lighting signal. Thereby, the level of the image signal of each line is corrected, and the image signal is obtained from which the level change caused by a change of lighting brightness is removed.
However, in an environment in which a special auxiliary lighting is not used for the image scanner, output voltages in a lighting light quantity detection signal and an image signal are small, and the adverse influence by noises is easily exerted.
Therefore, for a sample hold output on which sample holding has been performed on the signals to detect flickers, offset components are superimposed on minute output signals. If the offset component is superimposed only to the image signal or only to the flicker detection signal, it cannot usually be eliminated by a simple division, and remains as the influence of the flicker.
Furthermore, in the system of detecting lighting light radiating to the original surface as a flicker removing signal, a nonlinearity of photoelectric conversion property of the linear image sensor becomes a problem. Specifically, the linear image sensor does not necessarily correspond linearly to the output signal for the brightness, and the nonlinear property becomes relatively remarkable particularly in a dark area with a small signal. An example of the photoelectric conversion property of the linear image sensor having this nonlinearity is shown in FIG. 9.
In FIG. 9, for the photoelectric conversion property having the nonlinearity shown by a curve 100, in a usual brightness, it is assumed that the linear image sensor has a linear photoelectric conversion property curve 101, but in an area with a small light quantity, the property is curved, the origin of linear property deviates, and a sensor output voltage drops. Moreover, even in the light quantity of zero, the voltage does not become zero, and the offset component by noise is superimposed to the photoelectric conversion property.
Here, it is assumed that the lighting light radiated to the original is guided to the linear image sensor to detect a flicker component, the flicker detection light quantity fluctuates in the range of L1 to L3, while the quantity of light reflected from the original, that is, the light quantity for generating the image signal fluctuates in the range of L2 to L4. In this case, the sensor output voltage corresponding to the flicker detection light quantity fluctuates in the range of V1 to V3 which is converted by the substantially linear property. However, since the influence of the origin deviation of the linear property is large in the range of L2 to L4, the ratio of V2 and V4 is largely different from the ratio of L2 and L4.
Therefore, even when the ratio of L1 and L2 is the same as the ratio of L3 and L4, the ratio of V1 and V2 differs from the ratio of V3 and V4. Specifically, the linear relation to the brightness collapses between the lighting light quantity detection signal and the image signal. Even if the image signal is divided, the flicker component cannot be removed.
An object of the present invention is to, when an image is taken by an indirect type image scanner, effectively remove a transverse streak image attributed to flicker phenomenon of a ceiling lamp, even if light taken into a linear image sensor from an original image surface becomes insufficient and the nonlinearity of photoelectric conversion property of the linear image sensor becomes remarkable, or even if noise of a signal processing system raises a problem, and to enhance the image quality of a read image even when the image is read in a dark environment or at a high speed.
To achieve the object, according to the present invention, there is provided an image input device which comprises a linear image sensor provided with a plurality of linearly arranged light receiving elements for outputting an image signal of a linear image formed on the light receiving element; an image forming optical system for forming an image of a linear area on an original onto a row of the light receiving elements of the linear image sensor; a sub-scanning mechanism for moving the linear area formed into the image on the linear image sensor to scan the entire image of the original by the linear image sensor; a lighting light quantity detecting section for outputting a lighting light quantity signal indicative of a voltage corresponding to a lighting light quantity radiating to the original; a correction voltage adding section for adding a correction voltage to at least one of the lighting light quantity signal and the image signal to eliminate an influence by nonlinearity of a photoelectric conversion property of the linear image sensor and the lighting light detecting section; and a correcting section for eliminating a voltage fluctuation by a fluctuation of the lighting light quantity of the image signal with or without the correction voltage added thereto based on the lighting light quantity signal without or with the correction voltage added thereto.
Furthermore, the correction voltage adding section comprises either one of a first voltage adding circuit for adding a first voltage to the lighting light quantity signal to eliminate the influence by the nonlinearity of the photoelectric conversion property of the linear image sensor and the lighting light detecting section, and a second voltage adding circuit for adding a second voltage to the image signal to eliminate the influence by the nonlinearity of the photoelectric conversion property of the linear image sensor and the lighting light detecting section. The correcting section comprises a division circuit for using the lighting light quantity signal or the lighting light quantity signal with the first voltage added thereto as a denominator input, and using the image signal with the second voltage added thereto or the image signal as a numerator input to perform division.
According to the present invention, the voltage is added to the lighting light quantity signal or the image signal to eliminate the influence by the nonlinearity of the photoelectric conversion property of the linear image sensor and the lighting light detecting section, and the lighting light quantity signal without or with the voltage added thereto is used as the denominator input and the image signal with or without the voltage added thereto is used as the numerator input to perform the division. Thereby, when the light from the original is so weak that the image signal output with a sufficient voltage cannot be obtained from the linear image sensor, when a plus or minus offset voltage is superimposed as a noise to the image signal output from the linear image sensor, and further even when the input light quantity from the original is minute to a degree such that the nonlinearity becomes remarkable in the photoelectric conversion property of the linear image sensor, the influence of the offset voltage or the nonlinearity of the photoelectric conversion property of the linear image sensor is removed, a virtual black voltage obtained by extending the linear property of a portion in which the image signal is generated can be set to zero, and the flicker component included in the image signal can be removed by the division.
Moreover, in the image input device according to another aspect of the present invention, the correction voltage adding section comprises a first voltage adding circuit for adding a first voltage to the lighting light quantity signal to eliminate the influence by the nonlinearity of the photoelectric conversion property of the linear image sensor and the lighting light detecting section, and a second voltage adding circuit for adding a second voltage to the image signal to eliminate the influence by the nonlinearity of the photoelectric conversion property of the linear image sensor and the lighting light detecting section. The correcting section comprises a division circuit for using the lighting light quantity signal with the first voltage added thereto as a denominator input, and using the image signal with the second voltage added thereto as a numerator input to perform division.
According to the present invention, the voltage is added both to the lighting light quantity signal and the image signal to eliminate the influence by the nonlinearity of the photoelectric conversion property of the linear image sensor and the lighting light detecting section, and the lighting light quantity signal with the voltage added thereto is used as the denominator input and the image signal with the voltage added thereto is used as the numerator input to perform the division. Thereby, not only a virtual black voltage obtained by extending the linear property of a portion in which the image is formed and the image signal is generated, but also a virtual black voltage obtained by extending the linear property of a photoelectric converting element used for detecting the lighting light quantity can exactly be set to zero, and the flicker correction by the division is appropriately processed. Even if the detected lighting light quantity has a brightness such that the nonlinearity of the linear image sensor raises a problem, the flicker removing correction can securely be performed. Furthermore, even when the use environment changes and the offset component deviation attributed to natural light is generated in the flicker detection signal and the image signal, the flicker components can be removed.